Photolithography is commonly used during formation of integrated circuits associated with semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a radiation-sensitive material (such as, for example, photoresist) associated with a semiconductor wafer. The radiation-patterning tool can be referred to as a photomask or a reticle. The term “photomask” is traditionally understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses a pattern for either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized with their modern meanings so that the terms interchangeably refer to tools that encompass patterns for either a portion or an entirety of a wafer. Specifically, the term “reticle” will be used to generically refer to radiation-patterning tools that have patterns for either a portion of a wafer or an entirety of a wafer.
Reticles contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light-transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. As discussed previously, the wafer is provided with a layer of radiation-sensitive material (such as, for example, photosensitive resist material, which is commonly referred to as photoresist). Radiation passes through the reticle onto the layer of photoresist and transfers a pattern defined by the reticle onto the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
A prior art method of forming a reticle is described with reference to FIGS. 1–5. Referring initially to FIG. 1, a construction 10 is provided. Construction 10 comprises a substrate (or base) 12 having layers 14 and 16 formed thereover.
Substrate 12 of construction 10 will typically comprise, consist essentially of, or consist of material substantially transparent to radiation which is ultimately to be passed through a reticle formed from construction 10. Substrate 12 can, for example, comprise, consist essentially of, or consist of quartz.
Layer 14 of construction 10 comprises a material which attenuates radiation passed through a reticle formed from construction 10, and can be referred to as a radiation-attenuating layer. The material utilized for layer 14 will typically vary depending on the wavelength of radiation which is to be passed through the reticle. Typical materials consist essentially of, or consist of, MoSi, TiN, ZrO, SiNO, and TaHf, with the compositions being shown in terms of the elements contained therein rather than in terms of a particular stoichiometry of the elements.
The reticle formed from construction 10 will typically be fabricated to be utilized with either 157 nanometer wavelength radiation, 193 nanometer wavelength radiation, 248 nanometer wavelength radiation, or 365 nanometer wavelength radiation. The material utilized in layer 14 will typically be MoSi if the reticle is fabricated for utilization with 193 nanometer technology, and can be a different material if the reticle is fabricated for utilization with other technologies.
The specific wavelengths utilized with reticles correspond to specific wavelengths that can be generated with particular lasers. For instance, 193 nanometers corresponds to a wavelength which can be generated utilizing an ArF laser. The 157 nm, 193 nm, 248 nm and 365 nm technologies correspond to specific generations of radiations corresponding to specific generations of photolithography, with each subsequent generation being a shorter wavelength than the previous generation. A common tool utilized in photolithographic processing with reticles is a stepper, and the various generations of radiation are sometimes referred to as generations of stepper radiation.
Layer 16 of construction 10 typically comprises a material which is substantially totally opaque to the radiation which will ultimately be passed through a reticle formed from construction 10. Layer 16 will usually comprise chromium, and can be referred to as a chrome layer.
A layer 18 is formed over layer 16. Layer 18 can comprise a material sensitive to laser radiation and/or e-beam radiation, such as, for example, a photoresist. The radiation is utilized to pattern layer 18.
Referring next to FIG. 2, layer 18 is patterned into a series of structural elements 20, and the pattern from layer 18 is subsequently transferred to layer 16 with a suitable etch.
Referring to FIG. 3, layer 18 (FIG. 2) is removed, and the pattern from layer 16 is transferred to layer 14 with a suitable etch.
Referring to FIG. 4, layer 16 (FIG. 3) is removed. The remaining portions of layer 14 shown in the FIG. 4 view correspond to structural elements 22 formed over substrate 12. Typically, the chromium-containing layer 16 would only be removed from over portions of substrate 12, and would remain over other portions. Such is illustrated in FIG. 5, which shows a top view of the reticle construction 10 at the processing of stage of FIG. 4. Reticle construction 10 comprises an outer periphery 24 and an interior region 26. The patterned portion of the reticle is within the interior region 26, where a series of patterned elements 22 are formed. Chrome-containing layer 16 extends around peripheral region 24, yet has been removed from within interior region 26.
The reticle formed in accordance with the processing of FIGS. 1–5 can subsequently be utilized to pattern light during photolithographic processing. FIG. 6 illustrates reticle construction 10 utilized in a photolithographic process. The reticle 10 is shown inverted relative to the orientation of FIGS. 1–5. Radiation 30 is shown passing through reticle construction 10, and a graphical illustration of the transmittance of the radiation through the reticle construction is shown beneath the reticle construction. Construction 10 is shown to comprise two types of regions with a bar provided between construction 10 and the graph of transmittance. Specifically, construction 10 comprises a first type of region 32 corresponding to locations where structures 22 of material 14 are present, and a second type of region 34 corresponding to locations where structures 22 of material 14 are not present. Radiation 30 passes only through substrate 12 in locations 34 of construction 10, whereas the radiation 30 passes through both material 14 and structure 12 in the second regions 32 of construction 10.
The graphical illustration of transmittance occurring through various regions of construction 10 shows that approximately 100% transmittance occurs through second regions 34, and very little transmittance occurs through first regions 32. The radiation passing through first regions 32 is shifted substantially out of phase relative to the transmittance through second regions 34 (i.e. is shifted by about 180° relative to the radiation passing through second regions 34).
Typically the transmittance through first regions 32 will have an absolute value of less than 10%, with about 6% being common. Since the radiation passing through first regions 32 is out of phase relative to that passing through second regions 34, the transmittance through first regions 32 is shown having a negative value in the graph of FIG. 6. Although negative values of transmission are shown in FIG. 6 to emphasize a phase difference of radiation passing through the first regions of construction 10 relative to the second regions, it is to be understood that typically the percent transmission is expressed as an absolute value. In an effort to avoid confusion throughout this document and in the claims that follow, the claims will utilize the term “absolute transmission” to indicate when an absolute value of transmission is referred to. Additionally, it is noted that the light-restrictive properties of a material can be expressed as attenuation rather than as transmission. For purposes of interpreting this document and the claims that follow, attenuation is defined as being 100 minus the absolute value of percent transmission. Accordingly, the regions 34 of FIG. 6 have an attenuation of about 0, and the regions 32 of FIG. 6 have an attenuation of approximately 94. The attenuation of patterned, partially transparent masks utilized in prior art reticles (e.g., the patterned mask of material 14 of construction 10) is typically greater than 90%, and frequently is about 94%.